This invention relates to electronic parts, and more specifically to chip inductors.
Many different chip inductors have hitherto been introduced or brought into use. They are roughly divided into two types, those made by the lamination technique and those using rectangular magnetic cores. An advantage common to the both is smallness in size.
A few typical constructions will be described below with reference to the accompanying drawings. FIG. 1 shows a laminated chip inductor made by printing a coil-forming conductor 1 on each lamination of a magnetic (insulating) coating material 3 (the laminate being shown as having been sintered and therefore without inter-lamination seams) and repeating the run a number of times until a spiral conductor pattern, printed from lamination to lamination, forms a coil, covering the laminate with outermost layers to provide protection, and finally covering the both ends of the laminate with external-terminal electrodes 2 as terminals of the resulting coil. Inductors of this type are integrated in construction, small in size, and directly solderable to printed circuit boards and the like. Moreover, the structure requires no lead wire. In contrast with these and other advantages, they have a major disadvantage of a low self-resonance frequency with a large distributed capacity, because the conductor in the film form makes the inductor similar in construction to a laminated capacitor. Consequently, the dielectric loss is large and Q is lowered. Where a change in the inductance is desired the number of laminations must be changed correspondingly. This means an alteration in the thickness of the inductor according to the inductance required, which renders standardization of inductor sizes difficult. Furthermore, controlling the thickness of the magnetic coating films is so cumbersome that it often causes undesired variations of L and Q values. For the coil-forming conductor a precious metal, such as Ag or Ag-Pd alloy, must be used, and the thin-film conductor so formed naturally has high resistance to electric current. In addition, since the coil-forming conductor and its neighboring portions are all embedded in the magnetic material, the closed magnetic path or circuit causes early magnetic saturation and deteriorates the DC bias property of the product. Besides, there is a limitation to the number of turns in the coil formed by printing, and a high inductive intensity cannot be obtained.
FIG. 2 illustrates a conventional chip inductor using a rectangular magnetic core. This inductor is built by winding a wire 5 round recessed portions on four sides of a magnetic core 4 which is generally flat, rectangular parallelopiped recessed in the middle, extending the both ends of the wire forming the coil to the both ends of the rectangular core, connecting the wire ends or terminals to respective filmy electrodes 6 by means of solder 7, and then filling up the recesses with a resin 8 to protect the coil and finish the whole as a flat, rectangular parallelopiped. The product again has varied merits as a chip inductor and also shortcomings of large eddy current loss and very low Q (about 2 to 10) because of the rather broad electrode films on the both ends of the magnetic core. The depth B of the recesses in the thickness direction (vertical as viewed in FIG. 2) is necessarily less than the depth A of the recesses in the width direction. As a consequence, the absolute number of turns of the wire cannot be increased beyond a certain limit and hence the inductance of the coil is limited. A further disadvantage of this type is the necessity of an extra, knotty step of molding an insulating resin around the coil for its protection.
FIG. 3 shows another example of ordinary chip inductor using a rectangular magnetic core. The core 9 consists of a square stem having on both ends two square flanges dissimilar in shape and size. On the square stem, or in the recesses between the flanges, a wire 10 is wound into a coil, the both ends 13 of the coil are pulled round the edges of the larger flange and secured to electrodes 15 on the reverse side of the flange by means of solder 14, and then a protective cap 12 is fitted onto the smaller flange and the stem. Here again the dimension widthwise and therefore the inductance of the coil are restricted. The exposed coil ends can be broken by contact with other parts, thus affecting the reliability of the device. Moreover, because the electrodes to act as external terminals cannot be secured to any points other than at the bottom, the inductor is difficult to attach onto a printed circuit board and, if attached, the bond fails to possess adequate strength.